Simplify services.downloader.update_url_credentials()
Return Proxy-Authorization header in get_auth_info_from_url() when proxy=True
Support `http_proxy` for downloads we control (Closes #39)

Curdling now reads the `http_proxy` variable from the environment and
uses the `urllib3` proxy mechanism to route all requests through the
informed proxy server;

Authentication is also supported. For that case, inform the user and
password in the proxy URL. E.g.:

  $ http_proxy=http://user:passwd@server:port curd install flask
More tests and small improvements for the downloader